1. Field of the Invention
The present invention relates to microelectromechanical systems (MEMS) and, in particular, relates to the fabrication of MEMS structures.
2. Discussion of the Related Art
Microelectromechanical Systems (MEMS) components are being progressively introduced into many electronic circuit applications and a variety of micro-sensor applications. Examples of MEMS components are electromechanical motors, radio frequency (RF) switches, high Q capacitors, pressure transducers and accelerometers. In one application, the MEMS structure is an accelerometer having a movable component that, in response to acceleration, is actuated so as to vary the size of a capacitive gap. Accordingly, the electrical output of the MEMS structure provides an indication of the strength of the external stimulus.
One method of fabricating such components, often referred to as surface micro-machining, uses a sacrificial layer, such as silicon dioxide, that is deposited and bonded onto a substrate, such as single crystal silicon which has been covered with a layer of silicon nitride. A MEMS component material, for example polycrystalline silicon, is then deposited on the sacrificial layer, followed by a suitable conductor, such as aluminum, to form an electrical contact with the ambient environment. The silicon layer is then patterned by standard photolithographic techniques and then etched by a suitable reactive ion etching plasma or by wet chemistry to define the MEMS structure and to expose the sacrificial silicon dioxide layer. The sacrificial layer is then etched to release the MEMS component.
Several disadvantages are associated with fabricating a MEMS structure using a sacrificial layer. First, it requires an etching process that selectively etches the sacrificial layer without reacting with the other materials that will ultimately form the MEMS structure. This limits the materials that may be used when fabricating the MEMS structure. Additionally, the use of a sacrificial layer increases the amount of materials needed to form the MEMS structure, thereby adding cost to the fabrication process. Furthermore, an additional etching step is needed to remove the sacrificial layer, thereby further reducing the efficiency of the fabrication process. In particular, because the structure forming the movable MEMS element is disposed above the sacrificial layer, a significant amount of time is needed to completely undercut the sacrificial layer.
What is therefore needed is an improved method for fabricating a MEMS structure that avoids the inefficiencies associated with the use of a sacrificial layer.
The present inventors have recognized that a recess may be pre-etched in a substrate for a MEMS structure that facilitates release of the device.
In accordance with an aspect of the invention, a method of fabricating a MEMS structure includes attaching an etchable wafer to an upper surface of a substrate having a recess formed therein. The wafer includes a wafer portion from which a movable MEMS structure will be formed. The wafer is attached onto the substrate so that the wafer portion is positioned above the recess. Next, the wafer is etched downwards around the periphery of the movable structure to break through into the recess to release at least part of the movable structure from the substrate. This method therefore foregoes the need to perform substantial undercutting of a sacrificial layer.
The above aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, and not limitation, a preferred embodiment of the invention. Such embodiment does not define the scope of the invention and reference must be made therefore to the claims for this purpose.